1. Field of the Invention
The present invention is related to an image position aligning method, an image position aligning apparatus, and an image position aligning program for aligning a plurality of images with respect to each other.
2. Description of the Related Art
The energy subtraction process that utilizes the fact that amounts of attenuation of radiation that passes through subjects differ depending on the substance through which the radiation passes is conventionally known. The energy subtraction process employs two radiation images obtained by irradiating two types of radiation having different energies onto a subject. The energy subtraction process may be utilized to generate a soft tissue image, which is an image obtained of the thoracic region from which bones are removed, for example. Such a soft tissue image enables observation of patterns that appear within soft tissue, without the observation being impeded by the bones. Conversely, a bone image, from which soft tissue has been removed, may be generated, to enable observation of patterns that appear within bones, without the observation being impeded by the soft tissue.
In the case that the two radiation images obtained by the two types of radiation having different energies to be employed in the energy subtraction process are obtained by two imaging operations, high energy radiation and low energy radiation are irradiated onto a subject with an interval of several hundred milliseconds therebetween. If the subject moves between the two imaging operations, positional shifts occur, and artifacts will appear within an image (referred to as a subtracted image) generated by the subtraction process. In addition, these artifacts also appear in cases that the temporal subtraction process that obtains subtracted images that represent differences between two radiation images in temporal series obtained at different times is performed.
Various techniques for reducing such artifacts have been proposed. For example, U.S. Patent Application Publication No. 20080247626 discloses a technique for estimating local movements between two radiation images, in which ROI's (Regions Of Interest) are set within the two radiation images, and the ROI's are employed to perform positional alignment by performing matching. In this technique, the sizes of the ROI's within the images are reduced in a stepwise manner, to perform positional alignment of smaller structures in a gradual manner. Specifically, ROI's of a certain size are set at corresponding control points within two images to calculate an amount of positional shift. One of the two images is deformed to perform positional alignment based on the calculated amount of positional shift. The aligned images are employed to perform positional alignment employing ROI's of a smaller size. The above processes are repeatedly performed until a predetermined smallest size of the ROI's is reached. Thereby, positional alignment is performed among tissue included in the images in a gradual manner from structures in low frequency bands to structures in high frequency bands.
A plurality of structures, such as the heart, pulmonary blood vessels, ribs, and the diaphragm, respectively exhibit different three dimensional movements in the thoracic region of a human body. The plurality of structures overlap each other in the direction that a subject image is projected onto a radiation detector that detects radiation and outputs image signals. Here, for example, the heart includes a large amount of comparatively low frequency components, and the pulmonary blood vessels include a large amount of comparatively high frequency components. In the case that the technique disclosed in U.S. Patent Application Publication No. 20080247626 is employed, positional alignment is performed from structures in low frequency bands to structures in high frequency bands. Therefore, positional alignment is performed in a stepwise manner from the heart to the pulmonary blood vessels. However, in the technique disclosed in U.S. Patent Application Publication No. 20080247626, structures in low frequency bands are deformed along with the structures in high frequency bands when positional alignment of the structures in high frequency bands is performed. Therefore, in the case that two images include structures that exhibit different three dimensional movements such as the heart and pulmonary blood vessels, positional shifts occur in the heart, which has already been positionally aligned, when positional alignment of the pulmonary blood vessels is performed. If positional shifts occur in the heart in this manner, artifacts will be present in a subtracted image obtained by the energy subtraction process.